Modeling the Data Warehouse

Chapter 7

Data Warehouse Database Design PhasesDefining the business

model (conceptual model)

Creating the dimensional model (logical model)

Modeling summariesCreating the physical

model

Select a businessprocess

Physical modelPhysical model

2,3

Performing Strategic Analysis

Phase 1: Defining the Business Model

Select a businessprocess

�Performing strategic

analysis

�Creating the business

(conceptual) model

Creating the Business Model

Phase 1: Defining the Business Model Performing strategic analysis Creating the business (conceptual)model - Defining business requirements - Identifying the business measures - Identifying the dimensions - Identifying the grain - Identifying the business definitions and rules - Verifying data sources

Phase 1: Defining the Business Model

Creating the Business Model

Performing strategic analysis Creating the business (conceptual) model - Defining business requirements - Identifying the business measures - Identifying the dimensions - Identifying the grain - Identifying the business definitions and rules - Verifying data sources

Business Requirements Drive the Design Process

Typ e tit le h ere Typ e tit le h ere Typ e tit le h ere

Typ e tit le h ere

Primary input

Other inputsBusiness requirements

Existingmetadata

ProductionERD model

Research Nonrelationallegacy systems

Identifying Measures and Dimensions

The attribute variescontinuously: •Balance•United Sold•Cost•Sales

The attribute is perceived asa constant or discrete value:

•Description•Location•Color•Size

DimensionsMeasures

Determining Granularity

YEAR?

QUARTER?

MONTH?

WEEK?

DAY?

Identifying Business Rules

Location

Geographic proximity0 - 1 miles1 - 5 miles> 5 miles

Product

Type Monitor StatusPC 15 inch New

Server 17 inch Rebuilt 19 inch Custom

None

Time

Month>Quarter>Year

Store

Store>District>Region

Creating the Dimensional Model

Identify fact tables - Translate business measures into fact tables - Analyze source system information for additional measures - Identify base and derived measures - Document additivity of measures Identify dimension tables Link fact tables to the dimension tables Create views for users

Dimension Tables

Dimension tables have the following characteristics:Contain textual information that represents the

attributes of the businessContain relatively static dataAre joined to a fact table through a foreign key

reference

Facts(units, price)

Channel

Time

Product

Customer

Fact Tables

Fact tables have the following characteristics: Contain numeric measures (metric) of the business May contain summarized (aggregated) data May contain date-stamped data Are typically additive Have key value that is typically a concatenated key

composed of the primary keys of the dimensions Joined to dimension tables through foreign keys that

reference primary keys in the dimension tables

Facts(units, price)

Channel

Time

Product

Customer

Fact table

Dimension tables

Star Schema Model

Central fact tableRadiating

dimensionsDenormalized

model

Sales Fact TableProduct_idStore_ idItem_idDay_idSales_dollarsSales_units

Product TableProduct_idProduct_desc

Store TableStore_idDistrict_id

Time TableDay_idMonth_idYear_id

Item TableItem_idItem_desc

Star Schema Model

Easy for users to understandFast response to queriesSimple metadataSupported by many front end toolsLess robust to changeSlower to buildDoes not support history

Snowflake Schema Model

Sales Fact TableProduct_idStore_ idItem_idDay_idSales_dollarsSales_units

Product TableProduct_idProduct_desc

Store TableStore_idDistrict_id

Time TableDay_idMonth_idYear_id

Item TableItem_idItem_desc

District TableDistrict_id

District_desc

Dept TableDept_id

Dept_descMgr_id

Mgr TableDept_idMgr_id

Mgr_name

Snowflake Schema Model

Direct use by some toolsMore flexible to changeProvides for speedier data loadingMay become large and unmanageableDegrades query performanceMore complex metadata

Country State County City

Using Summary Data

Phase 3: Modeling summariesProvides fast access to precomputed

dataReduces use of I/O, CPU, and memoryIs distilled from source systems and

precalculated summariesUsually exists in summary fact tables

Designing Summary Tables

AverageMaximum

TotalPercentage

Units Sales($) Store

Product A Total

Product B Total

Product C Total

Summary Tables Example

SALES FACTSSales$ Region Month10,000 North Jan 9912,000 North Feb 9911,000 South Jan 9915,000 West Mar 9918,000 South Feb 9920,000 North Jan 9910,000 East Jan 992,000 West Mar 99

SALES BY MONTH/REGIONMonth Region Tot_Sales$Jan 99 North 41,000Jan 99 East 10,000Feb 99 South 40,000Mar 99 West 17,000

SALES BY_MONTHMonth Tot_SalesJan 99 51,000Feb 99 40,000Mar 99 17,000

Summary Management in Oracle8i

Summary advisor

Summaryrecommendations

Spacerequirements

Summaryusage

Region

State

City

Product Time

Salessummary

Using Time in the Data Warehouse

The Time Dimension

Time is critical to the data warehouseA consistent representation of time is

required for extensibility

Sales factTime

dimension

Where should the element of time be stored?

Creating the Physical Model

Phase 4: Creating the Physical Model Translate the dimensional design to a physical

model for implementation Define storage strategy for tables and indexes Perform database sizing Define initial indexing strategy Define partitioning strategy Update metadata document with physical

information

Physical Model Design Tasks

Define naming and database standardsPerform database sizingDesign tablespacesDevelop initial indexing strategyDevelop data partition strategyDefine storage parametersSet initialization parametersUse parallel processing

Using Data Modeling Tools

Tools with a GUI enable definition, modeling, and reporting

Avoid a mix of modeling techniques caused by: - Development pressure - Developers with lack of knowledge - No strategyDetermine a strategyWrite and publish formallyMake available electronically

Spreadsheets

CASE tools Paper andpencil

Summary

This lesson discussed the following topics:

Creating a business modelCreating a dimensional modelModeling the summariesCreating a physical model

Select among businessprocesses

Physical modelPhysical model

2,3

Business model

Dimensional model